In recent years, with the increased throughput and the higher integration of semiconductor chips, moves to use copper (Cu) with low electric resistivity and high electromigration resistance as a metallic material for forming an interconnection circuit on a semiconductor substrate, instead of aluminum or aluminum alloy, have become noticeable. A copper interconnection of this type is generally formed by filling fine recesses formed on the surface of the substrate with copper. Methods for forming the copper interconnection include CVD, sputtering, and plating.
FIGS. 62A to 62C show an example to form a copper interconnection by copper plating in the sequence of steps. As shown in FIG. 62A, an insulating film 2 of SiO2 is deposited on a conductive layer 1a on a semiconductor substrate 1 having formed a semiconductor device. A contact hole 3 and a trench 4 for an interconnection are formed in the insulating film 2 by lithography and etching technology. A barrier layer 5 of TaN or the like is formed on the contact hole 3 and the trench 4, and a copper seed layer 7 is further formed thereon as a power supply layer for electroplating.
As shown in FIG. 62B, copper plating is applied to the surface of a semiconductor substrate W to fill copper into the contact hole 3 and the trench 4 of the semiconductor substrate 1 and also deposit a copper film 6 on the insulating film 2. Then, the copper film 6 and the barrier layer 5 on the insulating film 2 is removed by chemical mechanical polishing (CMP), thus making the surface of the copper film 6 filled into the contact hole 3 and the trench 4 for an interconnection lie flush with the surface of the insulating film 2. In this manner, an interconnection composed of the plated copper film 6 is formed as shown in FIG. 62C.
FIG. 63 shows the entire constitution of a substrate processing apparatus for performing the above series of interconnection formation steps in a clean room. In the clean room, an insulating film forming device 10, a lithography and etching device 12, a barrier layer forming device 14, a copper seed layer forming device 26, a copper plating device 18, and a CMP device 20 are housed. The substrate W having the insulating film 2 formed by the insulating film forming device 10 is accommodated into a substrate cassette 22, and transported to the lithography and etching device 12 for a subsequent step. The substrate W having the contact hole 3 and the trench 4 for an interconnection formed in the lithography and etching device 12 is transported, while being housed in the substrate cassette 22, to the barrier layer forming device 14 for a subsequent step. The substrate W thus processed in the respective devices is transported, while being accommodated in the substrate cassette 22, to subsequent steps, whereby the series of interconnection formation steps are sequentially performed.
FIG. 64 schematically shows a conventional general configuration of a copper plating device for use in the above type of copper plating. This plating device includes a cylindrical plating tank 602 opening upward and holding a plating liquid 600 inside, and a rotatable substrate holder 604 adapted to detachably hold a substrate W, such as a substrate, so as to face downward, and disposing the substrate W at a position at which it closes the upper end opening portion of the plating tank 602. Inside the plating tank 602, a flat plate-shaped anode plate (anode) 606 immersed in the plating liquid 600 to serve as an anodic electrode is horizontally placed, and the seed layer of the substrate W is to serve as cathodic electrode. The anode plate 606 comprises a copper plate or a gathering of copper balls.
A plating liquid supply pipe 610 having a pump 608 mounted inside is connected to the center of the bottom of the plating tank 602. Outside of the plating tank 602, a plating liquid receptacle 612 is placed. Further, the plating liquid which has flowed into the plating liquid receptacle 612 is returned to the pump 608 through a plating liquid return pipe 614.
Because of this structure, the substrate W is held facedown at the top of the plating tank 602 by the substrate holder 604, and rotated in this condition. With a predetermined voltage being applied between the anode plate 606 (anodic electrode) and the seed layer of the substrate W (cathodic electrode), the pump 608 is driven to introduce the plating liquid 600 into the plating tank 602, whereby a plating electric current is flowed between the anode plate 606 and the seed layer of the substrate W to form a plated copper film on the lower surface of the substrate W. At this time, the plating liquid 600 which has overflowed the plating tank 602 is recovered by the plating liquid receptacle 612, and circulated.
Copper easily diffuses into a silicon dioxide film during a semiconductor manufacturing process to deteriorate the insulating properties of the silicon dioxide film, and causes cross contamination during the steps of transportation, storage and processing of the substrate. Copper may also contaminate the interior of the clean room.
In detail, the substrate having the copper seed layer formed thereon used to be transported, while being placed in the substrate cassette, to the copper plating device, and the substrate having the copper film formed in the copper plating device used to be transported, while being put in the substrate cassette, to the CMP device. Thus, copper particles and copper ions adhering to the substrate, which are very active and harmful to other processes, were likely to diffuse into the clean room.
When a plated copper film is deposited on the surface of the substrate by use of a copper electroplating device, a voltage between the center of the seed layer of the substrate and the anode differs from a voltage between the periphery of the seed layer of the substrate and the anode, because of the electrical resistance of the copper seed layer formed on the surface of the substrate. Thus, the film thickness of the plated copper film on the periphery of the substrate is greater than the film thickness of the plated copper film at the center of the substrate.
When the plated copper film thicker on the periphery than at the center of the substrate is polished by a polishing device, the plated copper film remains unpolished on the periphery of the substrate, or the plated copper film at the center is scraped excessively, which is a phenomenon called dishing.
The distance between the anode and the substrate may be fully lengthened to increase the electric resistance of the plating liquid itself, thereby diminishing the influence of the electric resistance of the copper seed layer. This measure can make the film thickness of the plated copper film more uniform, but leads to upsizing of the apparatus.